Although radiation therapy is a mainstay of cancer management, significant numbers of radiotherapy patients treated with curative intent still ultimately fail. Traditional methods of improving cancer therapy have focused primarily on achieving increased tumor cell kill. However, over the last decade another treatment approach has emerged. This strategy aims to impair the tumor's nutritional support system by target the tumor blood vessel network. Such Vascular Targeting approaches are based on the recognition that a continuously expanding vasculature is an essential requirement for tumor initiation, progression and metastasis. Vascular Disrupting Agents (VDAs) are designed to cause a rapid and selective vascular shutdown in tumors. The resulting ischemia produces rapid and extensive tumor cell kill. A main focus has been on agents that reversibly bind with tubulin and prevent microtubule assembly. Treatment with such VDAs has been shown to lead to extensive tumor necrosis in a wide variety of tumor models and to synergistic interactions with conventional anticancer treatments including radiotherapy. The latter has been attributed to the ability of these agents to selectively destroy central regions of tumors, areas widely believed to contain cell populations resistant to cytotoxic therapies. Lead VDAs have now entered clinical trials but new questions regarding their successful application have arisen. One of the issues to be addressed in this research program is whether the steps considered to ameliorate VDA-treatment associated hypertension might actually compromise the antitumor efficacy of these agents. Secondly we will examine whether knowledge of pathophysiologic information obtained using doses closer to those attainable in the clinic will allow the optimization of dose scheduling and drug delivery, leading to superior antitumor efficacy. The final aspect of this research proposal will explore the potential of utilizing the quantification of VDA treatment induced mobilization of circulating endothelial progenitor cells as potential biomarkers of VDA activity. Studies will be carried out with the current lead tubulin binding VDA CA4P and its promising second generation analog OXi4503 which has also entered patient trials. We believe that the proposed preclinical investigations will yield critical insights to the questions posed and provide valuable guidelines/suggestions for future clinical VDA initiatives in oncology.